The present invention relates to a semiconductor device and a process for producing the same. More particularly, the present invention pertains to a semiconductor device having a capacitor of large capacitance and high reliability and a process for producing such semiconductor device.
As is well known, one type of capacitor in which a silicon dioxide film is used as an insulator film is widely employed in various kinds of semiconductor memory.
As the packing density of semiconductor integrated circuits increases, the area for capacitors becomes considerably small. A reduction in the area for capacitors decreases capacitance, and this leads to a lowering in reliability of the semiconductor memory. For this reason, a means has already been proposed in which an oxide of a transition metal having a relatively large dielectric constant, e.g., Ta.sub.2 O.sub.5, is employed as a dielectric film of a capacitor in order to prevent lowering of capacitance.
For example, Japanese Patent Laid-Open No. 4152/1984 discloses a method wherein, after a tantalum oxide (Ta.sub.2 O.sub.5) film has been formed on a silicon substrate, a heat treatment is carried out in a wet oxidizing atmosphere to grow a silicon dioxide film at the interface between the tantalum oxide film and the silicon substrate, and then an upper electrode made of a refractory metal or a silicide of a refractory metal is formed on the tantalum oxide film to produce a capacitor.
However, the examination made by the inventor of this application has found that the capacitor produced by the above-described conventional method is unfavorably inferior in terms of long-term reliability, and the thickness of the silicon dioxide film formed between the tantalum oxide film and the silicon substrate is undesirably increased to lower the capacitance, thus considerably deteriorating effectiveness which is offered by employment of tantalum oxide as a dielectric film.
More specifically, although the above-described prior art has the advantage that formation of a silicon dioxide film between a silicon substrate and a tantalum oxide film enables a reduction in the defect density of the tantalum oxide film and consequently permits an improvement in dielectric strength, the capacitance per unit area is extremely decreased. Further, when the thickness of the silicon dioxide film exceeds 40 .ANG., it is impossible for the capacitor to obtain a long-term reliability which is superior to that of the silicon dioxide film since the degradation in dielectric strength with time of the capacitor in the case where a predetermined voltage is applied thereto depends on the durability of the silicon dioxide film at the interface between the silicon substrate and the tantalum oxide film.